High-Throughput Evaluation of Crystallinity and Phase Segregation of Organic Semiconductor Films Using Reflection Polarized Optical Microscopy

Crystallinity and morphology of organic semiconductor films are important parameters that impact their performance in electronic and optoelectronic devices, such as solar cells, light-emitting diodes, or thin film transistors. Characterization of the degree of crystallinity and morphology are typically conducted using techniques that either require synchrotron facilities (e.g., X-ray or neutron scattering), are destructive (e.g., electron microscopy or X-ray diffraction), or are relatively time-consuming (e.g., scanning probe microscopies). High-throughput, non-destructive characterization techniques for evaluating the degree of crystallinity are thus highly desired for in-line, automated device fabrication facilities.

Optical microscopy is a very simple, inexpensive, and non-destructive tool that can be used to rapidly diagnose the quality of thin films. Standard optical microscopy based on bright-field (BF) imaging provides very limited information about the crystallinity or morphology of organic semiconductor films. In contrast, polarized optical microscopy (POM), a method of imaging that employs a set of orthogonal polarizers in the illumination and collection paths of the microscope, can provide enhanced contrast for optically anisotropic materials, such as organic semiconductors. While POM has been widely employed to image the anisotropy of highly crystalline, highly oriented, or nanostructured organic molecules, there have been very limited applications of POM as a diagnostic tool for organic semiconductor films prepared using typical conditions for optoelectronic device applications – and those studies have been qualitative in nature [1-2].

In this work, we demonstrate that POM can be used as a high-throughput, non-destructive tool to evaluate the crystallinity and phase segregation of organic semiconductor blend films [3]. We outline the optimal imaging conditions required to maximize contrast for POM images of organic semiconductor thin films, which require using the reflection geometry with Köhler illumination and slightly uncrossed polarizers, having an uncrossing angle of ±3°. Using P3HT:PCBM as a model system, we quantitatively show that the contrast in POM directly correlates with 1) the degree of P3HT crystallinity and 2) the degree of phase segregation between P3HT and PCBM domains. We identify the origin of bright and dark domains in POM as arising from symmetry-broken liquid crystalline phases consisting of dark conglomerates. Finally, we discuss the methodology how reflection POM can be employed as a rapid diagnostic tool for automated device fabrication facilities.

[1] C. E. Petoukhoff and D. M. O’Carroll, “Optimization of PCDTBT Metal-Insulator-Metal Hole-Only Photodiodes", in International Conference on Hybrid and Organic Photovoltaics (HOPV22), Fundació Scito, Valencia, Spain, 2022.
[2] S. Alam, C. E. Petoukhoff, … and F. Laquai, “Influence of thermal annealing on microstructure, energetic landscape and device performance of P3HT:PCBM-based organic solar cells,” Journal of Physics: Energy, 6, 025013 (2024).
[3] R. Alzahrani, … F. Laquai, and C. E. Petoukhoff, “On the Use of Reflection Polarized Optical Microscopy for Rapid Comparison of Crystallinity and Phase Segregation of P3HT:PCBM Thin Films,” Macromol. Rapid Commun., (in press, 2024).